This invention relates to new silicic acid heteropolycondensates, to a process for their production and to their use as membranes and adsorbents.
Various membrane materials and adsorbents are known for the separation of mixtures of substances. These known membrane materials and adsorbents are being continuously developed with a view to improving their technical performance and economy. In the treatment of effluents, it has been possible for example by developing asymmetrical membranes to achieve the throughflow level required for commercial applications. On the other hand, there are still some unsolved problems in other fields of application. For example, known membrane materials, such as cellulose acetate membranes, are not particularly resistant to temperature and pressure and swell to a considerable extent in organic solvents. In consequence of the low resistance of these known membrane materials to temperature, pressure and solvents, their pore size is subject to continuous fluctuation in technical use and, as a result, can lead to non-reproducible results and to short service lives of the membranes.
Large numbers of adsorbents for purifying liquid or gaseous media are also known; for example active carbon and oxides having a high specific surface, such as aluminium oxide. With these known adsorbents, however, adsorption is generally non-specific and far from selective. In addition, the adsorption process is in many cases difficult to reverse at high adsorption levels, in other words desorption involves high energy consumption.
In addition, known adsorbents and membranes of which the surface has been chemically modified have the disadvantage that, in general, they only have a modifying monolayer at the surface with the result that they are extremely sensitive to mechanical and chemical influences, for example to hydrolysis.
It has now been found that certain silicic acid heteropolycondensates are particularly suitable for the production of temperature-, pressure- and solvent-resistant membranes for a variety of different separation problems. In granular or similar form, these silicic acid heteropolycondensates may be used as adsorbents even for highly specific separation which cannot be carried out satisfactorily, if at all, with known adsorbents. In addition, the silicic acid heteropolycondensates according to the invention are suitable for various other commercial applications.